A radio communication system compliant with LTE (Long Term Evolution) in 3GPP (Third Generation Partnership Project) is known. This radio communication system is configured by arranging a plurality of base stations so that each of the base stations communicates with a communication terminal (a mobile station) located within a communication area (referred to as a cell hereinafter) allocated to the base station.
The radio communication system uses the same communication band in each of a plurality of cells. Therefore, a difference between the level of a signal transmitted and received by a communication terminal (referred to as an edge terminal hereinafter) located on the border between cells to and from the base station of an own cell (a cell to which the edge terminal belongs) and the level of a signal (i.e., an interference signal) transmitted and received in an adjacent cell (a cell adjacent to the own cell) is small. Thus, there is a fear that the quality of a communication path (the communication path quality) between the edge terminal and the base station of the own cell becomes extremely low (deteriorates).
In order to address such a problem, there is a known technique called FFR (Fractional Frequency Reuse) aiming at suppression of signal interference between cells. The FFR is a technique of limiting allocation of a radio resource (a communication band and transmission power) in the adjacent cell in order to secure the quality of a communication path between the edge terminal and the base station of the own cell.
The FFR is classified into Static FFR in which a method for limiting allocation of a radio resource is not changed and Dynamic FFR in which a method for limiting allocation of a radio resource is changed.
At first, the outline of an operation of a radio communication system in which the Static FFR is applied to a downlink (a communication link for transmitting data from a base station to a communication terminal) will be described.
For the respective cells, the radio communication system sets priority bands that vary with the cell. Next, each communication terminal notifies communication path quality information representing the communication path quality to the base station. Based on the notified communication path quality information, the base station determines whether the communication terminal having notified the communication path quality information is a terminal (referred to as the edge terminal hereinafter) comparatively largely affected by signal interference from the adjacent cell or a terminal (referred to as the center terminal hereinafter) comparatively largely affected by signal interference from the adjacent cell.
After that, the base station allocates a communication band to be used for performing communication with the edge terminal from the set priority band. Moreover, the base station uses preset reference transmission power as transmission power to be used for performing communication with the edge terminal.
Further, the base station allocates a communication band to be used for performing communication with the center terminal from the whole communication band available in the cell. Furthermore, the base station uses limitation transmission power smaller than the reference transmission power, as transmission power to be used for performing communication with the center terminal.
Accordingly, it is possible to increase the communication path quality between the edge terminal and the base station (Non-Patent Document 1).
Next, the outline of an operation of a radio communication system in which the Dynamic FFR is applied to a downlink will be described. In this example, the radio communication system is equipped with three base stations 1 to 3 and seven communication terminals 11 to 13, 21, 22, 31 and 32 as shown in FIG. 1.
To each of the base stations 1 to 3, one cell is allocated. To be specific, a cell C1 is allocated to the base station 1, a cell C2 is allocated to the base station 2, and a cell C3 is allocated to the base station 3. Each of the base stations may be configured so that a plurality of cells can be allocated thereto.
Further, the communication terminals 11 to 13 belong to the cell C1 (i.e., a communication link for performing communication with the base station 1 is established). The communication terminals 21 and 22 belong to the cell C2. The communication terminals 31 and 32 belong to the cell C3. Herein, a case in which the communication terminals 12, 13 and 31 are edge terminals and the other communication terminals 11, 21, 22 and 32 are center terminals will be considered.
In this embodiment, a case in which the radio communication system divides a communication band (a system band) F0 available in the radio communication system into three partial bands F1 to F3, sets the partial band F1 as a priority band of the cell C1, sets the partial band F2 as a priority band of the cell C2 and sets the partial band F3 as a priority band of the cell C3 as shown in FIG. 2 will be considered.
Each of the base stations 1 to 3 notifies a priority band set for the own cell to the adjacent cell in a case that the number of the edge terminals that are due to transmit data is equal to or more than a preset threshold (Non-Patent Document 2). Herein, a case in which the threshold is 2 will be described. Moreover, a case in which each of the base stations 1 to 3 is due to transmit data to all of the communication terminals belonging to the own cell, respectively, will be described.
For example, notification of the priority band to the adjacent cell is performed by using RNTP (Relative Narrowband TX Power) in the LTE downlink. Moreover, notification of the priority band to the adjacent cell is performed by using HII (High Interference Indication) in the uplink (Non-Patent Document 3).
The edge terminals belonging to the cell C1 are the communication terminal 12 and the communication terminal 13. Therefore, the base station 1 notifies the priority band F1 of the cell C1 to the base station 2 to which the cell C2 generating a signal comparatively strongly interfering with a signal transmitted and received by the communication terminal 12 is allocated and the base station 3 to which the cell C3 generating a signal comparatively strongly interfering with a signal transmitted and received by the communication terminal 13 is allocated, respectively.
On the other hand, both the communication terminals 21 and 22 belonging to the cell C2 are the center terminals. Therefore, the base station 2 does not notify the priority band of the cell C2 to any of the base stations. In a like manner, the edge terminal belonging to the cell C3 is the communication terminal 31 alone. Therefore, the base station 3 does not notify the priority band of the cell C3 to any of the base stations.
With reference to FIGS. 3 to 6, a method for allocating a radio resource used for performing radio communication between the base station and the communication terminal will be described more specifically. To the base station 1, the priority band of the adjacent cell is not notified. Therefore, the base station 1 sets transmission power to preset reference transmission power P0 for both the edge terminals and the center terminals as shown in FIG. 3. The reference transmission power P0 is the average value of maximum transmission power that is the maximum value of transmission power that can be simultaneously outputted by the base station 1, over the whole system band F0.
Further, as shown in FIG. 4, the base station 1 allocates communication bands to be used for performing radio communication between the base station 1 and the communication terminals to both the edge terminals and the center terminals from the system band F0.
That it to say, an edge terminal allocatable band FE, which is a communication band that can be allocated as a communication band used for performing radio communication between the base station 1 and the edge terminal, is the system band F0. In a like manner, a center terminal allocatable band FC, which is a communication band that can be allocated as a communication band to be used for performing radio communication between the base station 1 and the center terminal, is also the system band F0. In other words, the base station 1 does not limit a radio resource (both a communication band and transmission power) allocated to the communication terminal.
On the other hand, to the base station 2 and the base station 3, the priority band is notified by the base station 1, respectively. Therefore, as shown in FIG. 5, the base station 2 sets the transmission power for the edge terminal to reference transmission power P0, whereas sets the transmission power for the center terminal to limitation transmission power P1. The limitation transmission power P1 is smaller than the reference transmission power P0 by a preset transmission power difference ΔP.
Further, as shown in FIG. 6, the base station 2 allocates a communication band to be used for radio communication between the base station 2 and the center terminal from the system band F0. That is to say, the center terminal allocatable band FC that is a communication band allocatable as a communication band to be used for performing radio communication between the base station 2 and the center terminal is the system band F0.
On the other hand, as shown in FIG. 6, the base station 2 allocates a communication band to be used for performing radio communication between the base station 2 and the edge terminal, from the communication band except the priority band F1 of the cell C1 in the system band F0. That is to say, the edge terminal allocatable band FE that is a communication band allocatable as a communication band to be used for performing radio communication between the base station 2 and the edge terminal is a communication band composed of the partial band F2 and the partial band F3.
Like the base station 2, the base station 3 allocates a radio resource to the communication terminal belonging to the own cell.
Thus, in a case that the number of edge terminals due to transmit data is equal to or more than a threshold in a certain cell, it is possible by limiting allocation of a radio resource in an adjacent cell to suppress signal interference in a priority band set for the certain cell in the certain cell.    [Non-Patent Document 1] Bin Fan, et al., “A Dynamic Resource Allocation Scheme Based on Soft Frequency Reuse for OFDMA Systems,” IEEE 2007 International Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, IEEE, August 2007, pp. 121-125    [Non-Patent Document 2] Nortel, “Further Discussion on Adaptive Fractional Frequency Reuse,” May 2007, 3GPP R1-072376 (searched on the Internet on Feb. 15, 2011 <URL: http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1—49/Docs/R1-072376.zip>)    [Non-Patent Document 3] 3GPP TS 36.423 V8.7.0, September 2009, pp. 16, 48 and 49
A case in which, as shown in FIG. 7, a radio communication system is equipped with three base stations 1 to 3 and twelve communication terminals 11 to 13, 21 to 24, and 301 to 315 will be assumed. Moreover, a case in which the communication terminals 11 to 13 belong to the cell C1, the communication terminals 21 to 24 belong to the cell C2, and the communication terminals 301 to 315 belong to the cell C3 will be assumed.
In addition, a case in which the communication terminals 12, 13, 23, 24, 314 and 315 are edge terminals and the other communication terminals 11, 21, 22 and 301 to 313 are center terminals will be assumed. Moreover, a case in which each of the base stations 1 to 3 is due to transmit data to all of the communication terminals belonging to the own cell, respectively, will be assumed.
In the cell C3, the number of the communication terminals is comparatively large. Therefore, in the cell C3, a communication band allocated to each of the communication terminals is narrow (small). As the communication band becomes smaller, a throughput decreases. A throughput is the amount of data that a communication terminal receives in a unit time period. Thus, in the cell C3, a throughput for each of the communication terminals is comparatively small.
Furthermore, in a case that the abovementioned threshold is set to 2, the priority band is notified to the base station 3 by the base station 1 and the base station 2, respectively. Therefore, the base station 3 sets the transmission power for the center terminals to the limitation transmission power P1 smaller than the reference transmission power P0. Moreover, the base station 3 allocates a communication band to be used for performing radio communication between the base station 3 and the edge terminal, from the communication band (i.e., the partial band F3) except the priority band F1 of the cell C1 and the priority band F2 of the cell C2 in the system band F0.
Therefore, in the cell C3, a communication band allocated to each of the edge terminals further becomes smaller. Besides, in the cell C3, transmission power allocated to each of the center terminals further becomes smaller. As the transmission power becomes smaller, a throughput decreases. Thus, in the cell C3, there is a fear that throughputs for the respective communication terminals become extremely small.
Thus, the radio communication system described above has a problem that, in a cell in which the number of communication terminals due to transmit data is comparatively large, throughputs for the respective communication terminals become extremely small (deteriorate).
Accordingly, an object of the present invention is to provide a radio resource range setting device capable of solving the aforementioned problems.